Star formation in HI selected galaxies

Picking galaxies that contain neutral hydrogen (HI) turns out to be an excellant way to find those that are forming new stars. This is important because when surveying the evolution of the universe, astronomers must pick the star forming galaxies in a very inclusive manner. Star forming galaxies come in a wide range of shapes and sizes, and if the astronomer is not careful s/he may concentrate on one particular type of star forming galaxy which is easy to find, while missing out on less obvious ones which are still important.

The fuel for creating stars is the interstellar medium (ISM), and one of its key components is HI. The intent of the Survey for Ionization in Neutral Gas Galaxies (SINGG) is to survey the star formation in galaxies selected by HI, which is like looking for fire where you know the wood is. The "fire" is recognized by Hα emission which indicates the presence of hot young stars which ionize a portion of the ISM.

The selection and observations worked very well, we found star forming galaxies covering a wide range of morphologies, as shown by the example images below. Virtually all known types of star forming galaxies were found, including a rare collisional ring galaxy. Moreover, a companion study shows that when these galaxies are used to estimate the total star formation rate in the local universe, the result is close to what other researchers are finding. This means that HI selection does not miss much star formation, at least compared to other methods.

One surprising result is that none of the 93 HI targets in the initial analysis is dormant (see histogram below) - all contain at least one star forming galaxy, with some HI targets corresponding to multiple galaxies. The explanation probably lies in the gravitational stability of the ISM. If there is sufficient mass in ISM it becomes gravitationally unstable and this leads to star formation. The galaxies in this sample all have a few tens of millions of solar masses of HI. Apparently this much HI results in the formation of some new stars. Although in some cases the number of stars formed is rather small (a few dozen).


HIPASS J0005-28 (ESO409-IG015): A small high surface brightness blue compact dwarf galaxy. (D = 11 Mpc, M_HI = 1.9x10⁸ M_sun, size = 400pix = 8.9Kpc).	Detection Histogram: Each box represents a target HI selected source, the dots within the boxes represent the Hα emitting galaxies found at the target's position.		HIPASS J0209-10 (HCG 16): the four interacting spirals in Hickson Compact Group 16 represent the largest number of ELGs in in a single HIPASS source. From right to left they are NGC833, NGC835, NGC838, NGC839. At least two of these galaxies are blowing spectacular glactic winds out their minor axes. (D = 54 Mpc, M_HI = 2.0x10¹⁰ M_sun, size = 1150pix = 130Kpc).

HIPASS J0043-22 (IC1574): a low surface brightness dwarf irregular galaxy, which has star formation limited to a few faint HII regions. (D = 5 Mpc, M_HI = 3.5x10⁷ M_sun, size = 500pix = 5.1Kpc).	HIPASS J0145-43 (ESO245-G005): a nearby irregular galaxy with bright billowy HII regions. The overall morphology is similar to the Large Magellanic Cloud. (D = 4 Mpc, M_HI = 3.8x10⁸ M_sun, size = 1000pix = 9.2Kpc).	HIPASS J0317-41 (NGC1291): A face-on SB0 (barred lenticular) galaxy containing faint HII regions tracing an outer ring 29 Kpc in diameter, and whispy diffuse HII concentrated towards its nucleus (which is saturated in Hα resulting in an apparent hole in the image). (D = 11 Mpc, M_HI = 2.6x10⁹ M_sun, size = 1850pix = 43Kpc).	HIPASS J1018-17 (NGC3200): A moderately inclined Sbc galaxy having one of the highest HI masses in the sample. (D = 53 Mpc, M_HI = 3.7x10¹⁰ M_sun, size = 950pix = 105Kpc).
The above example images shows a portion of the field observed with the CTIO 1.5m telescope. In these images net Hα is displayed in red, narrow band images are displayed in green, and R band continuum in blue. This results in Hα emission appearind red to orange in hue, and stars to have a cyan or blue hue. Click on the images to see the example at full resolution. The caption notes the distance (D), HI mass (M_HI), and image size. Mpc and Kpc are short for mega-parsec and kilo-parsec, or about 3 million and 3 thousand light years respectively, while M_sun refers to the mass of the sun or about 2x10³⁰ kilograms.

Paper reference: Meurer et al. 2006, "The Survey for Ionization in Neutral Gas Galaxies: I. Description and Initial Results", ApJS, Volume 165, Issue 1, pp. 307-337, astro-ph/0604444.